This solution provides two ways -- first with a tabular environment and second with an adjustable tcolorbox
environment.
\documentclass{scrbook}%
\usepackage{tcolorbox}%
\usepackage{colortbl}%
\usepackage{xcolor}%
\usepackage[utf8]{inputenc}
\usepackage{blindtext}%
\newcounter{musterklausur}
\newlength{\kopfbreite}
\setlength{\kopfbreite}{0.9\textwidth}%
\newlength{\liniendicke}
\setlength{\liniendicke}{0.7mm}%
\newcommand{\musterklausurkopf}{%
\stepcounter{musterklausur}%
\begin{center}
\arrayrulecolor{black}%
\setlength{\arrayrulewidth}{\liniendicke}%
\begin{tabular}{p{\kopfbreite}}
\hline
\tabularnewline
\multicolumn{1}{c}{\Large \textbf{Musterklausur Nr. \themusterklausur}}\tabularnewline %\[3mm]
\tabularnewline
\multicolumn{1}{c}{\Large \textbf{Lösungsvorschlag}}\tabularnewline
\tabularnewline
\hline
\end{tabular}%
\end{center}%
}%
\newcommand{\musterklausurkopfalternativ}{%
\stepcounter{musterklausur}%
\begin{center}
\begin{tcolorbox}[width=\kopfbreite,arc=0mm,auto outer arc,boxsep=0.3cm,toprule=0.7mm,leftrule=0pt,bottomrule=0.7mm,rightrule=0pt,colframe=blue]
\begin{center}
\Large \textbf{Musterklausur Nr. \themusterklausur}\par
\vspace{\baselineskip}%
\Large \textbf{Lösungsvorschlag}
\end{center}
\end{tcolorbox}%
\end{center}
}%
\setlength{\parindent}{0pt}% No indentation
\begin{document}
\musterklausurkopf%
Aufgabe 1 % Use a problem/answer package for this!)
\blindtext
\newpage
\musterklausurkopfalternativ%
Aufgabe 2 % Use a problem/answer package for this!)
\blindtext
\end{document}

\maketitle
does the formatting rather than marking it up manually. But, as already pointed out, it is difficult to advise how best to do that without a Minimum Working Example.